Multiple Choice Test on C.1 Energy Sources

Which expression gives the energy density of a fuel if the values of its molar mass (M) density (d) and its standard enthalpy of combustion ΔH⦵c, are known.

(d x ΔH⦵c) ÷ M Explanation Dividing the enthalpy of combustion (in kJ mol−1) by the molar mass (in kg mol−1) gives the amount of energy given out by one kilogram of the fuel. If that value is then multiplied by the density (in kg m−3) it gives the amount of energy given out by one cubic metre of the fuel, which is the energy density in kJ m−3.

Which is the correct expression to calculate the percentage energy efficiency of an energy transfer?

(useful energy output ÷ total input energy) x 100 Explanation The energy efficiency compares the useful amount of energy obtained with the total amount of input energy expressed as a percentage.

What is the main advantage of using E10 gasohol (a mixture of 10% ethanol and 90% gasoline) rather than just gasoline to power an automobile?

Ethanol is a renewable source of energy. Explanation Gasohol produces less energy than pure gasoline both by mass and by volume. However ethanol can be produced from biomass and so countries that do not have natural oil reserves can produce their own ethanol making them less dependent on oil producing countries. Gasohol is generally less polluting than gasoline, but not considerably less, as it consists of 90% gasoline.

Power stations store the energy produced during periods of low demand that can be released quickly during peak demand periods. Which methods can be used to store this energy? I. Pumping water uphill to a higher reservoir II. Electrolysing water III. Charging rechargeable cells (batteries)

I, II and III Explanation All three methods can be used. Water can be released from a higher reservoir to generate hydroelectricity, the hydrogen formed from the electrolysis of water can be combusted (or used in a fuel cell) and rechargeable batteries can release electricity directly.

Which is a correct unit for specific energy?

J kg^−1 Explanation The specific energy of a fuel is the amount of energy released from a fixed MASS of the fuel.

Hydrogen has a considerably higher specific energy than gasoline or diesel and is also a much less polluting fuel. What is the main reason why hydrogen is not routinely used to power automobiles?

The critical temperature of hydrogen is well below normal operating temperatures so it cannot be stored as a liquid. Explanation The reaction between hydrogen and air needs to be explosive to fuel a conventional combustion engine. If a fuel cell is used to produce electricity directly then the risk of explosion can be controlled. Hydrogen can be obtained from several sources including the electrolysis of water as well as from oil or gas. Hydrogen cannot be liquefied (above − 240 oC) by pressure alone so it needs to be stored as a gas under pressure in bulky, heavy cylinders giving it a much lower energy density than gasoline or diesel.

It is sometimes said that the sun is the ultimate source of all our energy. For which source of energy is this not true?

geothermal Explanation The sun is responsible for the climate (wind) and the water cycle (hydroelectric) and natural gas is a fossil fuel formed by the action of sunlight on organic matter millions of years ago. Geothermal energy originates from the heat stored by the Earth during its formation and by the ongoing decay of radioactive isotopes in the Earth's core.

Which is a correct unit for energy density?

kJ m^−3 Explanation The energy density of a fuel is the amount of energy released from a fixed VOLUME of the fuel.

Which are both non-renewable sources of energy?

oil and natural gas Explanation Although some natural gas can be obtained from biomass and landfills, the majority of natural gas used, like crude oil, is a fossil fuel so non-renewable.

Which expression gives the specific energy of a fuel if the values of its molar mass (M) density (d) and its standard enthalpy of combustion ΔH⦵c, are known.

ΔH⦵c ÷ M Explanation Dividing the enthalpy of combustion (in kJ mol−1) by the molar mass (in kg mol−1) gives the amount of energy given out by one kilogram of the fuel which is the specific energy in kJ kg−1.